Biomass

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In the official arcade map Left 2 Die, biomass is used as a resource, and is obtained by killing special zerg units and destroying infested buildings. A science facility can be used to spend biomass in new units, structures and upgrades.[1]

In the the Heart of the Swarm campaign, biomass is used both for primary mission objectives and as optional objectives to gain levels for Kerrigan, effectively replacing mutagen from earlier in development.[2]